The present invention is directed to a method of processing a material by directing a laser beam against the material. More particularly, the invention is directed to a method of laser cutting a material which improves the quality of the cut while maintaining high cutting speed.
Laser cutting of materials such as copper and aluminum is difficult because of their high reflectivity, high thermal conductivity and high thermal diffusivity. In order to overcome these problems, it is necessary to generate the highest possible power density in the focused spot of the laser beam directed against the material to be cut by minimizing the focused spot diameter. This may be achieved by using a low order mode laser output such as TEM.sub.00 and optimizing the focusing optics to give minimum abberation.
A typical set of conditions for cutting aluminum is as follows:
Material thickness 0.090 inch PA1 Power 1,000 watts CW (continuous wave) PA1 Mode TEM.sub.00 PA1 Speed 80 inches/minute PA1 Kerf width 0.005 inch.
While using the above conditions, complete penetration can be achieved. However, the cut quality is poor due to a metallic burr which adheres strongly to the underside of the cut. In the case of 0.090 inch aluminum, this burr may be 0.020-0.040 inch in height. The main reasons for this burr are that the molten aluminum and its oxides are viscous and that the narrow-cut kerf does not allow sufficient flow of cutting gas down through the cut channel to eject the molten material.
It is known that this burr problem can be solved by broadening the kerf width by increasing the focused spot diameter of the laser beam directed against the material being cut. However, this solution is not very desirable because with an increase in the kerf width, the laser power required to maintain the power density necessary to overcome the reflectivity problems becomes very high, so that the cutting process is not possible or the efficiency thereof is very low. For example, if the kerf width is doubled, the laser power has to be increased by a factor of X4 to maintain the power density necessary to overcome the reflectivity problems. Increasing the kerf width is also disadvantageous, because the volume of dross that has to be ejected during cutting increases.
Conventionally, materials have been laser processed with a continuous wave (CW) or one of two types of pulsing. One of these types of pulsing is gated pulsing wherein the laser beam has a power waveform with respect to time as shown in FIG. 1 of the drawings. In gated pulsing, the power output is switched between two CW power levels, P.sub.1 and P.sub.2, P.sub.2 being the maximum CW output. The second type of pulsing used in CO.sub.2 laser processing is referred to as superpulsing or enhanced pulsing. The laser beam in superpulsing has a waveform with respect to time as shown in FIG. 2 of the drawings. The power output in superpulsing is switched between two CW power levels P.sub.3 and P.sub.4 as in gated pulsing with the addition of a pulse of peak power P.sub.5 which typically is three times the CW power level P.sub.4. This is referred to as the power enhancement factor. The average power is determined by the duty cycle, but is typically 20-50% less than the CW level P.sub.4. The second known approach to solving the burr problem in laser cutting of materials such as copper and aluminum is to use superpulsing. This does reduce the burr, but processing speeds are low, typically half the CW rate, due to the lower average power and the discontinuous nature of the process.
An object of the present invention is to provide an improved method of processing a material by directing a laser beam against the material which avoids the aforementioned problems and disadvantages with the known methods of laser beam processing of a material. More particularly, an object of the invention is to provide a method of laser cutting a material wherein the cut quality is improved by reducing the height of the burrs remaining on the cut material while maintaining high cutting speed.
These and other objects of the invention are attained according to the invention by providing the laser beam which is directed against the material during the processing of the material with a power waveform with respect to time which is characterized by a plurality of peak power pulses which exceed a maximum continuous wave (CW) power level at which the laser device can continuously operate and a predetermined CW power level which is sustained substantially constantly during the time between the peak power pulses. According to another feature of the invention, the CW power level directly follows each of the peak power pulses such that the average power level of the laser power waveform is equal to or greater than the predetermined CW power level.
In the disclosed preferred form of the invention for cutting, the laser beam is a low order mode laser output of a laser such as TEM.sub.00 which is focused to minimize the beam diameter or spot size directed against the material whereby a relatively high power density of the beam is obtained for processing the material. The power of the peak power pulses is preferably at least about 3 times that of the CW power level. The CW power level is maintained continuously between the peak power pulses according to the preferred embodiment. A duration of each of the peak power pulses is less than one half the time between the peak power pulses in this disclosed embodiment. The frequency of the peak power pulses can vary depending on process requirements, but is preferably at least one 1 kHz in the disclosed example of the method.
Aluminum, copper and stainless steel as well as other metals and non-metals can be cut according to the method of the invention at cutting speeds achieved with a CW laser beam with the burrs remaining on the cut edges after laser cutting being reduced to a height less than that which occurs with CW laser cutting. The method of the invention is also applicable for other types of material processing including welding, surface etching, machining, etc. with good results.
These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings which shown, for purposes of illustration only, one embodiment in accordance with the present invention .